1. Field of the Invention
This invention relates to a process for the preparation of block copolymers of the S-B-S type wherein S represents a polystyrene block and B represents a polybutadiene block. More specifically, this invention relates to a process in which hexane is used as the polymerization medium. Still more specifically, this process relates to the preparation of an initial small block of polydiene to give solubility or dispersibility to the initial block of polystyrene.
2. Related Prior Art
Block copolymers of the S-B-S type are known and are considered to be of great utility as self-curing elastomers or as modifiers for diene elastomers. These have a central core of polybutadiene and two terminal blocks of polystyrene. However, in view of the greater insolubility or non-dispersibility of the initial polystyrene block, a solvent such as an aromatic hydrocarbon or cyclohexane has been used in its preparation.
This problem is recognized in British Patent No. 1,130,770 which states on page 2, lines 60-65, "In each of these processes, however, a vinyl aromatic hydrocarbon is required to be first polymerized and therefore the polymerization solvent used must be wholly or mainly a naphthenic or aromatic hydrocarbon solvent."
Similar recognition of this solubility problem is indicated in U.S. Pat. Nos. 3,231,635, 3,265,765 and 3,427,364 wherein statements are made regarding the necessity to use aromatic or naphthenic solvents or mixtures of these with other solvents.
Brtish Pat. No. 1,130,770 effects polymerization in each step of a mixture of butadiene and styrene. Therefore in forming the initial block, the butadiene is described as giving hexane solubility to the resulting block polymer. As discussed in the following section taken from lines 32-56 of page 4, patentee discourages against the use of less than 10% butadiene for the initial block: "The monovinyl aromatic hydrocarbon content of the mixture of conjugated diolefin and monovinyl aromatic hydrocarbon, which is first polymerized by use of a monolithium hydrocarbon, is preferably from 15% to 90% by weight. If the content were less than 15% by weight, the elastic properties of the finally obtained copolymer would suffer. On the other hand, if the content were more than 90% by weight, the finally obtained copolymer would have lower thermal resistance; moreover, in this case the polymerization medium used would have to be wholly or mainly a naphthenic or aromatic hydrocarbon solvent. Further, owing to the lower solubility of the initially formed copolymer in paraffinic aliphatic hydrocarbon solvents it would be impossible to obtain a homogeneous solution or a slurry of sufficient dispersibility in such a solvent, so the subsequent additional copolymerization reaction after addition of a mixture of conjugated diolefin and monovinyl aromatic hydrocarbon would hardly proceed uniformly, whereby the physical properties of the finally obtained copolymer would suffer."
In every working example except one, the patentee uses at least 40% by weight and in some cases 50% and 65% of the conjugated diene in the initial monomer mixture. In only one example (Ex. 7) does the patentee use less than 40% and in that example he uses 20% isoprene and 80% styrene in the initial monomer mixture. Therefore patentee has not demonstrated that monomer mixtures having 10-15% by weight of conjugated diene will form an initial polydiene block to give hexane solubility.
When a high concentration of diene is present in a monomer mixture it is known that the initial block of polymer is a polydiene block as will be the case when 40-65% of diene is initially present. However, when the concentration of the diene is lower, that is 15% or less of diene and 85% or more of styrene, the tendency for random copolymerization of diene and styrene increases and the tendency is even stronger as the concentration of diene falls below 10% by weight. Therefore, while concentrations of 40-65% diene will give an initial block of polydiene that will give hexane solubility, the use of 15% or less of diene is much more likely to give an initial, more difficultly soluble random copolymer block.
British Pat. No. 1,412,584 has modified the process of preparing block copolymers so that heptane may be used as the polymerization medium, by making first a block of t-butyl-styrene and describes this first block as a thermoplastic polymer of 10,000 to 100,000 molecular weight to give solubility to the growing block copolymer. The second or middle block of polybutadiene is then added and finally a block of polystyrene. The resulting block copolymer is poly(t-Bu-styrene)-polyubutadiene-polystyrene.
Because of its lower boiling point and the resultant energy savings in the use of hexane as compared to benzene and cyclohexane, it is desirable to have a process designed to permit the use of hexane in preparing the S-B-S block copolymers. Moreover, the use of a process conducted in hexane makes it adaptable for use in equipment designed for the preparation of polybutadiene in hexane. Obviously the substitution of a poly(t-Bu-styrene) block for a polystyrene block is not a desirable solution for this problem, principally because of the additional cost of t-butyl-styrene as compared to styrene and because of the differences in properties between the two types of blocks.